Transfemoral (i.e. above-knee) amputation is a significant cause of disability in the United States with approximately 31,000 new cases occurring each year. Transfemoral amputees may use an assistive device known as an artificial leg, or a prosthesis, to replace a missing natural leg.
Transfemoral prostheses have traditionally been mechanically passive devices, which are controlled by mechanical interaction with the subject's limb. A passive prosthesis uses springs, dampers, or other passive devices that do not provide external energy to power the prosthesis. Passive lower-limb prostheses provide a lower limb amputee or other user with a limited range of ambulation modes, such as standing, walking, ascending or descending ramps, or climbing up and down stairs. Transfemoral amputees who use passive prostheses have significantly impaired balance, walking symmetry, and metabolic energy efficiency. They also tend to have difficulty navigating more demanding terrain such as ramps or stairs. The use of an active transfemoral prosthesis, with the ability to generate positive mechanical power, greatly increases the number and nature of ambulation modes that can be restored to amputees.
Lower limb prosthesis users may desire to ambulate in several modes, including level walking, climbing stairs, descending stairs, climbing ramps, or descending ramps. Operation in multiple modes requires the prosthesis to be able to transition from one mode to another. The failure of a prosthesis to transition prevents the user from moving appropriately and can result in harm to the user. For instance, if a lower limb prosthesis user wishes to transition from walking to climbing stairs, the prosthesis must be raised so that its foot raises above the first step. Failure of the prosthesis to transition appropriately can result in the device hitting the stairs, potentially damaging the prosthesis and causing injury to the user through a fall or other event.